A mental state of unbridled, irrepressible arousal that one's faculties are rendered useless to his/her environment for a brief moment in time, or until said state of arousal has come to fruition.

Joshua: So what happened next?
David: She pulled out a dildo and went to town on herself.
Joshua: What did you do?
David: I got up and said, "Yo, I'm gonna roll out."
Joshua: What did she say to that?
David: Nothing. Nothing at all.
Joshua: Dude. She was in the Jack-zone.